1. Field of the Invention
The present invention generally relates to burn-in systems and burn-in control techniques, and more particularly, to a burn-in system and a burn-in control technique by which a monitored burn-in test can be performed on a large number of semiconductor devices. The present invention also relates to a semiconductor device production method utilizing the burn-in control technique.
A method of producing semiconductor devices generally comprises: a wafer processing step of forming circuits on a wafer; a dicing step of dicing the processed wafer into individual semiconductor chips; a chip mounting step of die-bonding and wire-bonding the semiconductor chips to a lead frame; and a resin encapsulating step of encapsulating the mounted semiconductor chips with resin. Reliability tests are then performed on the semiconductor devices. One of the known reliability tests is a burn-in test. In the burn-in test, the semiconductor devices are heated at a predetermined high temperature for a predetermined period of time so as to facilitate initial failures to identify semiconductor devices with a possibility of failure in an early stage.
A burn-in system is used to perform such a burn-in test. There are two types of burn-in systems. One of them has a monitor function to check the operating states of the semiconductor devices during the burn-in test, and the other has no monitor function. A burn-in system having the monitor function is called a monitored burn-in system, and one having no monitor function is called a dynamic burn-in system.
The monitor function is essential for the determination of the quality of semiconductor devices. Having the monitor function, the monitored burn-in system can readily check whether semiconductor devices are in a proper energized state and are supplied with proper input signals. The monitored burn-in system is becoming more common as a means to improve the reliability of the burn-in test.
2. Description of the Related Art
FIG. 1 shows the structure of a conventional burn-in system 1. The burn-in system 1 comprises a burn-in chamber 2, a drive unit 3, a monitor unit 4, a burn-in chamber control unit 5, and a counter unit 6.
The burn-in chamber 2 adjusts its inner temperature to a desired temperature, and accommodates burn-in boards to which semiconductor devices are attached. A burn-in test is carried out on the mounted semiconductor devices in the burn-in chamber 2.
The drive unit 3 is connected to each of the burn-in boards accommodated in the burn-in chamber 2, and supplies operating current and operating signals to each of the semiconductor devices via the respective burn-in boards. The monitor unit 4 monitors the operations of the semiconductor devices driven by the drive unit 3, and supplies the monitor results to the counter unit 6.
The burn-in chamber control unit 5 controls the inner temperature of the burn-in chamber 2 to a desired temperature, and also stops the burn-in test upon receipt of a burn-in termination instruction issued from the counter unit 6. The counter unit 6 calculates a failure rate based on quality data of each semiconductor device supplied from the monitor unit 4 and the number of semiconductor devices accommodated in the burn-in chamber 2. When the failure rate becomes lower than a predetermined reference value, the counter unit 6 issues the burn-in termination instruction to the burn-in chamber control unit 5, thereby stopping the burn-in test. Here, a parameter is the number of semiconductor devices which work properly during a simple preliminary test among the semiconductor devices mounted in the burn-in chamber 2.
FIG. 2 is a graph showing the relationship between a burn-in time (abscissa axis) and a failure rate (ordinate axis) in a burn-in test performed on semiconductor devices. This graph is generally known as a bath-tub curve. As can be seen from the graph, the failure rate caused by the burn-in test is high at the beginning and decreases with time (an initial failure period). The failure rate then reaches a value xcex0 and stays in the vicinity of the value (a chance failure period). As the burn-in test continues, the failure rate rapidly increases at one point (wear-out failure period). According to the bath-tub curve, the point where the burn-in test moves on to the chance failure period from the initial failure period can be considered the beginning of a stable period. Accordingly, it is ideal to end the burn-in test at the beginning of the stabilized period.
In view of this, the counter unit 6 outputs the burn-in termination instruction based on two parameters, a shape parameter (m) and the failure rate (xcex(t)). The shape parameter (m) and the failure rate (xcex(t)) are calculated based on quality data and parameters supplied from the monitor unit 4 using a Weibull function. When the shape parameter(m) and the failure rate (xcex(t)) both become smaller than respective predetermined reference values, the burn-in test is stopped. When the shape parameter (m) and the failure rate (xcex(t)) are both equal to or larger than the respective predetermined reference values, the burn-in test continues automatically.
Normally, a Weibull function is expressed as:
F(t)=1xe2x88x92exp[xe2x88x92(t/xcex7)m](t greater than 0, xcex7 greater than 0)xe2x80x83xe2x80x83(1)
wherein xcex7 is a scale parameter. According to the Weibull function, the failure rate function is expressed as:
xcex(t)=(mxc3x97tmxe2x88x921)/xcex7mxe2x80x83xe2x80x83(2)
Using an acceleration factor xcex2, the failure rate function is also expressed as:
xcex7(t)=(mxc3x97tmxe2x88x921)/(xcex7mxc3x97xcex)xc3x9710E9)xe2x80x83xe2x80x83(3)
Here, the shape parameter (m) is determined from the inclination of an approximate line of a failure occurrence point plotted on a Weibull probability paper. When the shape parameter (m) is smaller than 1, the failure rate is decreasing. When the shape parameter (m) is equal to or larger than 1, the failure rate is increasing. The acceleration factor xcex2 is a coefficient for comparing the acceleration in the xe2x80x9clifetimexe2x80x9d, and varies between semiconductor devices.
The burn-in termination instruction is issued when the shape parameter (m) is smaller than a predetermined reference value (m0) and the failure rate (xcex(t)) is smaller than a predetermined reference value (xcex0). The timing of the issuance of the burn-in termination instruction is determined based on both the shape parameter (m) and the failure rate (xcex(t)), for the following reasons.
If the timing of the instruction issuance is determined from either the shape parameter (m) or the failure rate (xcex(t)) alone, in one case the burn-in termination instruction is issued when the shape parameter (m) is 0 and the failure rate is larger than the predetermined reference value (xcex0), as indicated by an arrow B in FIG. 2. In another case, the burn-in termination instruction is issued when the shape parameter (m) is larger than 0 and the failure rate is smaller than the predetermined reference value (xcex0), as indicated by an arrow C in FIG. 2. The arrows B and C indicate situations where the failure rate has not decreased (i.e., the initial failure period has not ended). If the burn-in termination instruction is issued at such times as indicated by these above cases, the accuracy and reliability of the burn-in test is reduced.
Furthermore, there is a limit to the number of semiconductor devices accommodated in one burn-in system (mounting capacity number). However, a large number of semiconductor devices are manufactured at once to improve the production efficiency. The semiconductor devices collectively manufactured need to be subjected the burn-in test at the same time. Therefore, when the number of semiconductor devices to be subjected to the burn-in test is larger than the mounting capacity number, the semiconductor devices are mounted to a plurality of burn-in systems. Each of the burn-in systems performs the burn-in test to provide Weibull judgements.
In a case where the mounting capacity for one burn-in board is 216 and one burn-in system can accommodate 48 burn-in boards, for instance, the parameter, or maximum capacity, is 10,368. If the number of semiconductor devices is larger than the parameter, or maximum capacity, a plurality of burn-in systems perform the burn-in test independently of each other, each independently stopping the burn-in test when a result calculated in accordance the Weibull function is smaller than a predetermined reference value.
By this conventional method, however, the timing of the termination of the burn-in test is different between the burn-in systems. The timing variation is due to the difference in number of semiconductor devices, and to the difference in failure rate between the burn-in systems.
In a case where one of the burn-in systems has completed the burn-in test, the semiconductor devices can be removed from the burn-in system. However, it is necessary to wait for the other burn-in system(s) to finish the burn-in test, because the semiconductor devices collectively manufactured need to be processed in the next production procedure. This adversely affects the efficiency of the entire production procedures including the reliability test.
As mentioned before, to calculate the failure rate using the Weibull function, it is necessary to determine the parameter, which is the total number of semiconductor devices subjected to the burn-in test. However, in the conventional method, the parameter is limited to the mounting capacity number of one burn-in system, and the failure rate of all the semiconductor devices collectively manufactured cannot be calculated using the Weibull function and the total number of the semiconductor devices as the parameter.
This causes an inconvenient situation where the failure rate in one burn-in system is very high while the failure rate in another burn-in system is very low. As the failure rate differs between the burn-in systems, it is impossible to determine an accurate failure rate of all the semiconductor devices collectively manufactured.
A general object of the present invention is to provide burn-in systems and burn-in control techniques in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a burn-in system and a burn-in control technique by which a burn-in test can be efficiently and accurately performed on a large number of semiconductor devices. Another specific object of the present invention is to provide a semiconductor device production method utilizing the burn-in system and the burn-in control technique.
The above objects of the present inventions are achieved by a burn-in system comprising:
a plurality of burn-in devices, each of which calculates a parameter indicating the number of mounted semiconductor devices, and generates measurement data indicating quality of the semiconductor devices collectively subjected to a burn-in test; and
a counter terminal which determines a total parameter from the parameters sent from each of the burn-in devices, calculates a failure rate based on the total parameter and measurement data also sent from each of the burn-in devices, and stops the burn-in test of the burn-in devices when the failure rate reaches a predetermined reference value.
The above objects of the present invention are also achieved by a burn-in control technique including the steps of:
mounting a plurality of semiconductor devices in each of the burn-in devices;
determining a parameter that is the number of semiconductor devices mounted in each of the burn-in devices;
generating measurement data indicating quality of the semiconductor devices by subjecting the mounted semiconductor devices to a burn-in test;
transmitting the parameter and the measurement data obtained from each of the burn-in devices to a counter terminal;
calculating a total parameter from the parameters sent from each of the burn-in devices;
calculating a failure rate based on the total parameter and the measurement data; and
terminating the burn-in test of each of the burn-in devices when the failure rate reaches a predetermined reference value.
Since the above burn-in system has a plurality of burn-in devices, each of which can perform a burn-in test, semiconductor devices beyond the capacity of one burn-in device can be mounted to the plurality of burn-in devices and subjected to the burn-in test. Each of the burn-in devices performs the burn-in test collectively on a plurality of semiconductor devices to obtain the quality data of each of them. Here, the number of semiconductor devices mounted to each burn-in device can be obtained as a parameter.
The counter terminal receives the parameter and measurement data from each of the burn-in devices, and determines the total parameter by adding up the parameters sent from the burn-in devices. Since the counter device is independent of the burn-in devices, it can determine the total number of semiconductor devices mounted to the burn-in devices. The counter unit also calculates the failure rate based on the total parameter and the measurement data sent from the burn-in devices. As the calculated failure rate varies with the total parameter, it represents the failure rate of all the semiconductor devices mounted to the burn-in devices.
The counter terminal also stops the burn-in test when the failure rate reaches a predetermined reference value. Since the failure rate obtained as above has a high accuracy, the burn-in test can be stopped at the proper time. Also, the counter terminal can stop all the burn-in devices at once, because the burn-in test is terminated based on the failure rate of all the semiconductor devices mounted to the burn-in devices. Thus, the burn-in test can be performed at higher efficiency.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.